Distributed or “cellular” radio communication systems are typically comprised of a number of cells, with each cell corresponding roughly to a geographical area. Each cell has an associated base station (BS) which is a local central site providing access to the communication system to a number of radio transmitter/receiver units (user terminals (UTs)) within the cell.
In such distributed wireless communication systems, a user terminal generally has the capability to move in and out of various cells. While a base station typically handles traffic exchange with a number of user terminals within its cell, a user terminal typically exchanges traffic with a single base station at a time. However, when communication quality degrade swith respect to a particular base station(s) and improves with respect to one or more other base stations (e.g., due to changes in the RF environment, movement of the user terminal away/toward cells, etc.), the user terminal should be able to handover “active communication” to the base station providing better communication.
FIG. 1 depicts a typical distributed voice and/or data network topology, in accordance with the prior art. As shown, a number of base stations 102, 104, and 106 are distributed geographically, such that each provides a cell coverage area (or simply cell) 112, 114, and 116, respectively. In turn, the cells 112, 114, and 116 each has an associated cell boundary 118, 120, and 122, respectively, simplistically depicted circularly in FIG. 1.
As a user terminal 108 shown in cell 114 (and therefore assumed to be in active communication, i.e., exchanging voice and/or data traffic, with the associated base station 104 which may be coupled to a larger voice and/or data network, such as PSTN or the internet) moves in a direction 110 away from the base station 104 and toward the base station 106 and its associated cell boundary 122, handover will typically take place—i.e., the user terminal's active communication session will transfer to the new base station 106. As such, if the user is, for example, exchanging data with a device coupled to the internet (which in turn is coupled to the base stations shown in the network 100 of FIG. 1), such exchange will not be interrupted due to the user terminal's movement away from the cell coverage of one station; the exchange or session will be handed over to another base station.
Handover schemes are typically based on optimizing a cost function (C) that depends on one or more parameters, such as the received signal strengths (RSSI) from one or more base stations within the communication range of the user terminal, or visa versa, at a given time. As such, the scheme can be implemented by the user terminal, the base station, or a combination thereof. Most common schemes involve user terminal-centric handover, i.e., measurements (e.g., RSSI) made by the UT are solely used in the handover cost function. In such schemes, the user terminal periodically samples transmissions (e.g., the broadcast message(s)) from a set of base stations, including any with which it is in active communication, and optimizes the cost function to select the best base station.
Because of the fluctuations (e.g., due to shadowing and other real-world RF effects) in the power of each base station as experienced by a user terminal, the handover cost function C usually should include a margin, sometimes known as a hysteresis margin. In other words,C=(SA−Si)+h,  (1)where h is the hysteresis (sometimes also referred to as the hysteresis margin or factor), SA is the RSSI of the current (or also sometimes referred to as the active) base station, and Si is the RSSI of an ith candidate base station. When C<0, then handover should be initiated.
The hysteresis margin h is included in the cost function to prevent the user terminal from frequent “ping ponging” between two or more base stations due to the power fluctuations of their transmissions. This particular need for h is further illustrated and described in connection with FIG. 2.
FIG. 2 is a graphical representation of the theoretical and practical signal strengths of two base stations experienced by a user terminal as the user terminal moves away from one of the base stations with which it is in active communication and toward the other base station. The signal strength of each base station's transmissions as received at the user terminal 108 is represented by the vertical axis as a function of distance as the user terminal 108 moves with a velocity 110 away from base station 104 and toward base station 106.
Ignoring the practical power fluctuations described above, the pathloss associated with the base stations 104 and 106, given the user terminal 108's described motion, could be represented by the lines 204 and 206, respectively, in which case there would be no need for h. In this ideal case, once the two lines cross and the RSSI associated with base station 106 exceeds that associated with the base station 104, handover to base station 106 can occur smoothly without the ping pong effect alluded to above.
However, because of a somewhat chaotic RF environment, where shadowing, scattering, etc., cause power fluctuations and a non-linear pathloss to occur, the actual pathloss associated with base stations 104 and 106 and experienced by the receiving user terminal 108 might appear more like the irregular graphical lines 202 and 208, respectively. And as evident from the regions 210 and 212, for example, there may be instances where the RSSI of one base station exceeds the other, but then shortly later, the situation may reverse, contributing to a back-and-forth or “ping pong” handover effect.
To prevent a ping pong handover effect from occurring, some prior techniques select a relatively large h to use in the handover cost function C. However, if the hysteresis factor h is fixed at too large a value, then communication quality may degrade too much before handover takes place. On the other hand, if the hysteresis is too small, then there may still exist a potential for the above-described ping pong effect to occur. To illustrate, the behavior of the pathloss lines 202 and 208, for example, near the intersection of their ideal counterparts 204 and 206, respectively, show that (1) if h is fixed value that is selected too small, it will likely contribute to an undesired ping pong effect and (2) if h is fixed value that is selected to be too large, handover may be delayed until communication quality degrades beyond a desirable level.
Because the velocity of the user terminal influences signal strength (e.g., RSSI) calculations when windowing is used, some handover techniques use a variable measuring window in which to take signal strength measurements, and then change the window length relative to the calculated velocity of the user terminal. Unfortunately, this technique suffers from drawbacks. First, estimating the velocity of the user terminal is a relatively difficult task. Second, using a variable window length renders implementation, for example in a digital signal processor (DSP), impractical since buffers need to change.
In addition, though the cost function (C) primarily is a function of signal strength, there may be instances where selecting a base station whose C is optimized relative to other base stations may not provide optimum performance. Unfortunately, most prior techniques have not addressed using other selection criteria for selecting a base station.
Thus, what is needed is a method and apparatus for facilitating base station selection, including handover, to overcome the above-mentioned limitations of prior systems and methods.